Attenuator for coaxial transmission lines



G. L. FERNSLER ATTENUATOR FOR. COAXIAL TRANSMSSION LINES June 4, 1946.

Filed Nov. 2l, 1942 mss PATENT OFFICE 2,401,296 ATTENUATOR FOR COAXIAL TRANSMISSION LINES George Fernsler, Lawrenceville, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application November 21, 1942, Serial'I No. 466,408

9 claims. (ci. ris-44) 1 2 This invention relates generally to transmisattenuator section, and means including a. varision circuits, and particularly to a variable atable conducting iris for adjusting the attenuation v i tenuator for a, coaxial transmission line in which of said attenuator.

the attenuation is proportional to the cross-sec- 'I'he invention will be described by reference to tional area of a variable mechanical aperture in the accompanying drawing, in which Figure lis said attenuator. a cross-sectional view of one embodiment of the YVarious attenuators have been utilized heretoinvention;` Figure 2 is a cross-sectional view of fore for coaxial transmission line circuits, but in a second and preferred embodiment of the inattenuator has involved the problem of comprovention; and Figure 3 is a side elevational view mising between linearity of attenuation and reac- ,10 of the adjustable iris element. Similar reference tion upon the static characteristics of the transnumerals are applied to similar elements throughmission line. out the drawing.

The instant invention contemplates the use of Referring to Fig. 1, a section of coaxial cable a novel and relatively simple transmission line includes an outer cylindrical conductor l and an section which includes -a pair of enlarged elecinner hollow cylindrical or solid conductor 2,| of 4 y.

trodes forming a gap in the inner conductor of conventional proportions and design. A section the line and means including an adjustable meof the center conductor 2 is removed and enlarged chanical conducting iris for adjusting the attenconducting electrodes 3, 4 are attached to the uation of the transmission line section without respective ends of the center conductor to form affecting substantially the surge impedance of a gap 5. This gap should preferably be substanthe line. tially shorter Athan the distance between the inner Briefly, the invention utilizes the capacity couconductor 2 and the outer conductor l. An adpling between the two terminating electrodes of justable conducting iris 6, the aperture diameter the central conductor ofthe line and means inof which may be varied by means. of a knob 1 cluding the adjustable iris for varying the capacattached to a shaft 8 which extends outsideof itance between these electrodes. the outer conductor l.- is disposed preferably nor- Among the objects of the invention is to promal to the transmission line axis and substantially vide a new and improved method of and means at the mid point between the electrodes 3, 4. vThe for providing a variable -attenuator for a coaxial adjustable iris 6, therefore, forms a shielding transmission line. Another object of the invenelement which varies the capacitive coupling betion is to provide lan improved method of and tween the electrodes 3, 4.' As the aperture of the means for adjusting linearly the attenuation in iris 6 is enlarged by adjusting the position of the' y a coaxial transmission line section by means of knob 1, the capacitive coupling between the elecan adjustable conducting iris disposed normal to trodes 3, 4 is increased and vthe attenuation of the transmission line axis, A further object oi 35. the line is decreased accordingly. By forming the invention is to provide an improved method v the electrodes 3, I in a semi-prolate spheroidal of and means for adjusting linearly theattenushape, the attenuation may be made substantially ation oi' a coaxial transmission line by means of linear with respect to the size of the iris aperture. an adjustable iris normal to the axis of said line, The aperture adjustment should preferably be wherein the surge impedance of the transmis- `.io limited to dimensions which will not aect masion line remains substantially constant irreterially the line surge impedance.

spective of the attenuation inserted in the line The apparatus just described has the principal circuit. Another object of the invention is to disadvantage that the surge impedance varies provide improved means for attenuating linearly considerably in the neighborhood of the elecin a coaxial transmission line including a section 4 .trodes 3, 4, due to the change in the ratio of the of coaxial cable inserted in said line, wherein the diameter of the outer conductor of the line to central conductor of said section is enlarged to the diameter of the inner conductor of the line in form two semi-prolate spheroidal electrodes disthis region. posed coaxially with said line, and separated by The device illustrated in Fig. 2 overcomes the a gap which is small in relation to the distance disadvantage described heretofore for the device between the inner and outer conductors of the of Fig. l, by providing an enlarged tapered porline. A still further objectof the invention is to tion 9 in the outer conductor l of the transmisprovide an improved variable j linear attenuator sion line. Preferably, the ratio of the diameters for a coaxial transmission line which includes of the outer and inner conductors should remain two semi-prolate spheroidal elements forming a 'substantially constant throughout the section of gap in the inner conductor of said line, surthe line which includes the Asemi-prolate spherounded by enlarged portions of the outerconroidal electrodes. VIt should be understood that ductor oi said line, whereby the ratio of the dithis relation will be only approximate, since it is ameters of the outer and inner conductors realso desirable to form both the inner and outer mains substantially constant throughout the conductors in such a manner that there are no 3 sharpedges or curved sections of small radii. Therefore, the shapes of the inner and outer conductors of the line will be varied somewhat by streamlining the conductors at the point where the capacitive electrodes are inserted in the inner conductor. Otherwise, the device illustrated in Fig. 2 may be identical to the device of Fig. 1, described heretofore, and the adjustment thereof is accomplished in the same manner.

In addition, rings of carbon, or other resistance material i0, may be inserted adjacent the electrodes 3, 4. normal to the line axis, to provide terminating resistors equal to the line surge impedance.

It should be understood that the adjustable iris 8 may be in the general form of a continuously adjustable camera iris, or it desired, may be in the form of an adjustable stop aperture device wherein a plate having different apertures of fixed dimensions is rotated to bring the desired aperture between the spheroidal elements 8, 4.

The camera iris type is illustrated in Fig. 3

wherein the vanes II are pivoted at the points i2, and the dimensions of the aperture I6 are adjusted by turning the outer ring i3.

Thus the invention described comprises a simple and eillcient means for providing linear attenuation of a coaxial transmission line without ai'- fecting seriously the surge impedance of the line at the point where the attenuator is inserted.

I claim as my invention:

1. An attenuator comprising a coaxial high frequency transmission line which includes two cooperative semi-prolate spheroidal conducting elements disposedwithin the outer conductor of said line and interposed coaxially between the ends of the inner conductor of said line to form a gap therein constituting capacitive coupling between the two portions of said inner conductor, and an 'apertured diaphragm disposed between said elements within said gap, the conformation of said spheroidal elements providing substantially linear variation of attenuation with diaphragm aperture size.

2. Apparatus of the type described in claim l including means for varying Athe cross-sectional area of said diaphragm aperture.

3. An attenuator comprising a coaxial high frel quency transmission line which includes two cooperative semi-prolate spheroidal conducting elements disposed coaxially within the outer conductor of saidline and interposed between the ends of the inner conductor of said line to form a gap therein, said gap and elements providing capacitive coupling between the two portions of said inner conductor, and an apertured diaphrgm disposed between said elements within said gap normal to the axis of said conductors, the conformation of said spheroidal elements providing substantiallylinear variation of attenuation with diaphragm aperture size.

4. An attenuator comprisingv a coaxial high frequency transmission line which includes two cooperative semi-prolate spheroidal conducting elements disposed coaxially within the outer conductor of said line and interposed between the ends of the inner conductor of said line to form a gap therein, said gap and elenents providing capacitive coupling between the two portions of said inner conductor, and an apertured electrical shield diaphragm disposed between said elements within said gap normal to the axis of said conductors. the conformation of said spheroidal elements providing substantially linear variation of attenuation with diaphragm aperture size.

These rings may be omitted. if desired.`

4 e 5. Apparatus of the type describedin claim 4 including means comprising gradually expanded portions of said inner and said outer conductor adjacent each of said elements for minimizing changes in the surge impedance of said line at said attenuator.

6. yA substantially linearly variable attenuator comprising a coaxial high frequency transmission line which includes an outer conductor having a gradually expanding and gradually contracting portion, two cooperative semi-prolate spheroidal conducting elements disposed coaxially within said gradually expanding and gradually contracting portions of said outer conductor of said line and interposed between the ends of the inner conductor of said line to form a gap therein, said gap and elements providing capacitive coupling between the two portions of said inner conductor, an apertured conducting diaphragm disposed between said elements within said gap, and means for varying the cross-sectional area of said diaphragm aperture, the conformation of said spheroidal elements providing substantially linear variation of attenuation'with adjustment of said diaphragm aperture area.

7. A substantially linearly variable attenuator comprising a coaxial high frequency transmission line which includes an outer conductor having a gradually expanding and a gradually contracting portion, two cooperative semi-prolate spheroidal conducting elements disposed coaxially within said gradually expanding and gradually contracting portions of said outer conductor of said line and interposed between the ends of the inner conductor of said line to form a gap therein, said gap and elements providing capacitive coupling between the two portions of said inner conductor. said gap being substantially shorter than the shortest distance between said outer conductor and said elements, an apertured conducting diaphragm disposed between said elements within said gap normal to the axis of said conductors. and means for varying the cross-sectionalarea of said diaphragm aperture, the conformation of said spheroidal elements providing substantially linear variation of attenuation with adjustment of said diaphragm aperture area.

8. A substantially linearly variable attenuator substantially shorter than the shortest distance between said outer conductor and said elements, an apertured conducting diaphragm disposed between said elements within said gap normal to the axis of said conductors, and means for varying the cross-sectional area oi' said diaphragm aperture uniformly with respect to said axis to provide'a linear relation between said area and the attenuation of said attenuator.

9. Apparatus of the type described in claim 8 including annular resistive means interposedbetween the conductors of said attenuator adjacent said elementsto provide terminating impedances of substantially the value-oi' said line impedance.

GEORGE L. FERNBLER. 

